A couple of things...
That valve should have a black plastic cap on the single nipple end, which encompasses the twin electrical connector, AND a little filter that fits over the open nipple. The nipple on that end, acts as a vent/bleed when the valve is not energized.
Here is a Diagram taken from my 1984 GM Factory Service Shop Manual.
Everyone should have a copy of the relevant year GM manual for the "10/20/30 Light Truck" as it has the most complete info out there, and is invaluable. I found my scrappy copy on Ebay for $20.
It seems, your solenoid is mounted on the thermostat housing which is correct. Mine is not!
As I have to understand what each emissions item does and how it works, I dug into this component, and what it does.
It seems that a signal is sent from the Distributor Tach output between 1700rpm and 1890rpm to the"Engine Speed Switch", which should be mounted on the rear part of the left hand valve cover...
And here is mine, bottom lh side of pic. This was before I refitted my emissions correct Quadrajet.
This switch then energizes the solenoid to open, sending vacuum to a the "Throttle Lever Actuator", a vacuum pod on the carb, holding the throttle open.
I believe this holds the throttle open to stabilize the mixture upon deceleration.